1. Field of the Invention
This invention relates generally to a method for calibrating and operating an accelerometer device. More particularly, this invention relates to methods of improving the processes and accuracy of measurements and calibration of accelerometer parameters.
2. Description of the Prior Art
Conventional techniques for carrying out accelerometer measurements and calibrations still have technical difficulties and limitations. The accelerometers generally generate three types of output signals. The first type of output signal is an analog signal such as an output voltage. The second type of output signal is a digital pulse width modulation (PWM) signal. The PWM signal has time duration with a length that represent the duty cycle corresponding to the voltage of the analog signal. The third type of output signal is a sequence of binary digital pulse that represents the voltage of the analog signal. For the purpose of simplifying the explanations, the following discussions of calibration of accelerometers use examples of analog signals while the technical principles and descriptions are applicable to all three types of output signal.
An output voltage Voutput is generated from an accelerometer when an acceleration represented by a parameter “a” is detected along the axes of the accelerometer. The acceleration “a” can be calculated from output voltage Voutput as:
                              Acceleration          ⁢                                          ⁢          a                =                              (                                                            V                  output                                -                                  V                  offset                                                            V                sensitivity                                      )                    ·          g                                    (        1        )            
There are two important accelerometer parameters, namely Voffset and Vsensitivity employed to compute the acceleration “a” according to Equation (1). The parameter Voffset representing an output voltage when there is no acceleration, i.e., when acceleration “a”=0. The parameter “g” in Equation (1) represents the gravity acceleration and in the following equations, Vg represents the voltage output when the acceleration value of the accelerometer has a value of “g”. As discussed above, the output signal from an accelerometer can also be a duty cycle of a pulse according to a pulse width modulation process for output signal generation or a pulse stream representing the voltage of the analog signal. In the above Equation (1):Vsensitivity=Vg−Voffset  (2)Initially, a manufacturer of the accelerometer provides the values of these two parameters Voffset and Vsensitivity and the user of the accelerometer then applies the values of these two parameters and Equations (1) and (2) to measure and determine the accelerations according to the output voltages or the duty of the pulse width. However, the values of these two parameters Voffset and Vsensitivity drift gradually and become inaccurate for acceleration computations. Inaccuracies of acceleration measurements are generated due to the value drifts of these two parameters. More particularly, the general practice of the manufacturers now is to measure the values of Voffset and Vsensitivity of the accelerometer based on the output voltages of an accelerometer along either a horizontal or a vertical direction. A user of the accelerometer is however unable to repeat such measurements in order to recalibrate the values of Voffset and Vsensitivity. With such limitation, the user of an accelerometer has limited option but to continue to use an accelerometer with the built in values of the Voffset and Vsensitivity that continues to drift with time thus seriously affecting the accuracy and usefulness of the accelerometers.
Therefore, a need still exists in the art of accelerometer measurements, calibrations and operation to provide new and improved methods and processes to overcome the above-discussed difficulties and limitations.